1. Field of the Invention
The field of this invention relates to the recycle of phosphorus-vanadium-oxide catalysts prepared in an organic medium thus conserving up to ninety percent of the vanadium. These catalysts can be promoted by a transition element of the IV and V period of the periodic table. These catalysts are useful for the manufacture of maleic anhydride from n-butane.
2. Background
Maleic anhydride is of significant commercial interest throughout the world and is extensively used in the manufacture of alkyd resins. It is also a versatile intermediate for chemical synthesis. Consequently, large quantities of maleic anhydride are produced each year to satisfy these needs. The production of maleic anhydride by the catalytic oxidation of benzene and butane is well known and until recently, the principal method employed for the manufacture of maleic anhydride was by the air oxidation of benzene in the presence of certain heavy metal oxide catalysts. However, because of the inherent toxicity of benzene fumes, the trend has been to eliminate the utilization of benzene as a feedstock and newer facilities tend to utilize butane oxidation processes.
In general, catalysts proposed for the oxidation of butane to maleic anhydride have been based upon vanadium and phosphorus. In U.S. Pat. No. 3,293,268 it is disclosed that the oxidation of butane to maleic anhydride can be performed in the presence of a phosphorus-vanadium-oxygen containing complex catalyst. Though this catalyst is capable of oxidizing butane, it does not give sufficiently high yields. Yields of maleic anhydride of only 30 to 50 weight percent are reported. Various activators, stabilizers and promoters have been disclosed in the prior art to improve the yields of maleic anhydride. References include U.S. Pat. Nos. 3,867,411; 3,832,359; 3,888,886; 4,002,650; 4,147,661; 4,149,992; 4,151,116; 4,152,338; 4,152,339 and British Application No. 2,019,839A. While the aforementioned prior art tends to bring about some improvement in the performance of the phosphorus vanadium catalyst there remains much room for improvement, particularly from the standpoint of high conversion, yield and catalyst life.
The manufacture of phosphorus-vanadium-oxide catalysts in an organic medium leaves economically significant quantities of vanadium, phosphorus and catalyst components dissolved in the mother liquor. In the event co-metal is used, significant quantities of the co-metal are also dissolved in the mother liquor. The co-metals include zinc, molybdenum, zirconium, niobium, cerium, chromium, manganese, nickel and uranium. About one third of the most expensive catalyst ingredient, vanadium, remains in the mother liquor. It is an object of this invention to recover the catalyst material remaining in the mother liquor and then reuse it in the manufacture of additional quantities of the phosphorus-vanadium-oxide catalyst useful for the oxidation of butane to maleic anhydride. Thus saving ninety percent of the vanadium, the most expensive component of the maleic anhydride catalyst. Suitable the phosphorus vanadium catalysts are promoted by a transition element of the IV and V period of the periodic table. Suitable promoters include molybdenum, zinc, zirconium, niobium, cerium, chromium, manganese, nickel, and uranium.
Surprisingly, it is found that the catalytic material found in the residue may be directly reapplied to prepare another fresh phosphorus-vanadium-oxide catalyst or the fresh phosphorus vanadium oxide-promoted catalyst without any deleterious effect on the catalyst activity.
The recycled catalyst comprises a phosphorus-vanadium mixed oxide. The atomic ratio of the vanadium to phosphorus can suitably be in the range of about 1.0:1.0 to about 0.50:1.0, preferably in the range of about 0.90:1.0 to about 0.60:1.0. In the event a promoter is employed, the total atomic ratio of the promoter to vanadium advantageously is in the range of about 0.001:1 to about 0.100:1. It is preferred that the total atomic ratio of a promoter such as molybdenum, zinc, zirconium, niobium, cerium, chromium, manganese, nickel and uranium to vanadium should be in the range of about 0.01:1 to about 0.05:1. The atomic ratio of phosphorus to vanadium is suitably in the range of about 1.00:1 to about 1.90:1, preferably about 1.10:1 to about 1.70:1.
Catalysts recycled according to the invention may be made from an organic solvent system wherein vanadium pentoxide in the presence of the promoter is reduced with gaseous hydrogen chloride. Subsequent reaction of the vanadium-promoter oxide solution with orthophosphoric acid and removal of water of reaction by azeotropic distillation result in precipitation of a crystalline vanadium-phosphorus mixed oxide or the vanadium-phosphorus-promoter mixed oxide which may suitably be filtered from the mother liquor, dried and then employed as an oxidation catalyst for the manufacture of maleic anhydride from butane feedstock. Suitably, organic solvents are alcohols or mixtures of alcohols with aromatic hydrocarbons such as benzene and orthoxylene. Aliphatic alcohols are usually employed in the process and isobutanol is the preferred alcohol. The precipitation of the phosphorus-vanadium-oxide complex or the phosphorus-vanadium-promoter oxide is achieved by reducing the solubility of this complex in solution by employing a co-solvent. Precipitation can also be effected by reducing the temperature and removal of the solvent. The use of a co-solvent such as benzene or orthoxylene also functions to facilitate removal of excess water through azeotropic distillation. Precipitation of the phosphorus-vanadium-mixed oxide can suitably be effected by azeotropic distillation of the organic solvent and the water of reaction and subsequent evaporation of the organic solvent. The promoter may be added as a compound together with vanadium or separately. In the event the promoter is molybdenum or zinc, suitable molybdenum or zinc compounds comprise metallic molybdenum or zinc, molybdenum chloride, zinc chloride, molybdenum oxide, zinc oxide and most soluble molybdenum salts and zinc salts. The mother liquor from the preparation of these catalysts can be recycled and additional reactants added to form a new catalyst. Surprisingly this catalyst prepared from the mother liquor has the same activity as a new catalyst, yet by using the recycle process, up to 90 percent of the vanadium is recovered.
According to our process, the average valence of vanadium is in the range of about +3.8 to about +4.5. In the catalyst preparation according to our recycle of the mother liquor, various anhydrous phosphoric acids may be used including ortho-phosphoric, pyrophosphoric, triphosphoric or meta-phosphoric acid. The vanadium compound can be vanadium pentoxide, vanadium tetrachloride, vanadium trichloride, vanadium oxydichloride, vanadium oxytrichloride, vanadium tetroxide, vanadium oxalate, and most soluble vanadium complexes. Suitable vanadium compounds include: vanadium oxides such as vanadium pentoxide, vanadium trioxide and the like; vanadium oxyhalides such as vanadyl chloride, vanadyl dichloride, vanadyl trichloride, vanadyl bromide, vanadyl dibromide, vanadyl tribromide and the like; vanadium-containing acids such as meta-vanadic acid, pyrovanadic acid and the like; vanadium salts such as ammonium meta-vanadate, vanadium sulfate, vanadium phosphate, vanadyl formate, vanadyl oxalate and the like. However, vanadium pentoxide is preferred.
This invention also comprises a process for oxidizing butane to maleic anhydride by contacting it in the presence of oxygen with the phosphorus-vanadium-oxide catalyst or the phosphorus-vanadium promoter oxide catalyst prepared from the mother liquor. The oxidation of butane to maleic anhydride may be accomplished by contacting n-butane in low concentration in oxygen with the described catalyst. Air is entirely satisfactory as a source of oxygen, but synthetic manufactures of oxygen and diluent gases such as nitrogen also may be employed. Air enriched with oxygen may be used.
The gaseous feed stream to the oxidation reactors will normally contain air and about 0.2 to about 1.7 mole percent of n-butane. About 0.8 to 1.5 mole percent of n-butane is satisfactory for optimum yield of maleic anhydride for the process of this invention. Although higher concentrations may be employed, explosive hazards may be encountered. Lower concentrations of butane less than about one percent, of course, will reduce the total yield obtained at equivalent flow rates and, thus, are not normally economically employed. The flow rate of the gaseous stream through the reactor may be varied within rather wide limits, but preferred range of operations is at the rate of about 100 to 4000 cc of feed per cc of catalyst per hour and more preferably about 1000 to 2400 cc of feed per cc of catalyst per hour. Residence times of the gas stream will normally be less than about four seconds, more preferably less than about one second, and down to a rate where less efficient operations are obtained. The flow rates and residence times are calculated at standard conditions of 760 mm of mercury and at 25.degree. C. A variety of reactors will be found to be useful and multiple tube heat exchanger-type reactors are quite satisfactory. The tops of such reactors may vary in diameter from about one-quarter inch to about three inches, and the length may be varied from about three to about ten or more feet. The oxidation reaction is an exothermic reaction and, therefore, relatively close control of the reaction temperatures should be maintained. It is desirable to have the surface of the reactors at a relatively constant temperature and some medium to conduct heat from the reactors is necessary to aid temperature control. Such media may be Woods metal, molten sulphur, mercury, molten lead and the like, but it has been found that eutectic salt baths are completely satisfactory. One such salt bath is a sodium nitrate, sodium nitrite-potassium nitrate eutectic constant temperature mixture. An additional method of temperature control is to use a metal block reactor whereby the metals surrounding the tube act as a temperature regulating body. As will be recognized by a man skilled in the art, the heat exchanger medium may be kept at the proper temperature by heat exchangers and the like. The reactor or reaction tubes may be iron, stainless steel, carbon steel, nickel, glass tubes such as vycor and the like. Both carbon steel and nickel tubes have excellent long life under the conditions of the reaction described herein. Normally, the reactors contain a heat preheat zone under an inert material such as one-quarter inch Alundum pellets, inert ceramic balls, nickel balls, or chips, and the like present at about one-half to one-tenth the volume of the active catalyst present.
The temperature of reaction may be varied within some limits but, normally the reaction should be conducted at a temperature within a rather critical range. The oxidation reaction is exothermic and once reaction is underway, the main purpose of the salt bath or other media is to conduct heat away from the walls of the reactor and control the reaction. Better operations are normally obtained when the reaction temperature employed is no greater than 20.degree.-50.degree. F. above the salt bath temperature. The temperature of the reactor, of course, will also depend to some extent upon the size of the reactor and the butane concentration.
The reaction may be conducted at atmospheric, superatmospheric, or below atmospheric pressure. The exit pressure will be at least slightly higher than the ambient pressure to insure a positive flow from the reaction. The pressure of the inert gases must be sufficiently higher to overcome the pressure drop through the reactor.
Maleic anhydride may be recovered by a number of ways well known to those skilled in the art. For example, the recovery may be by direct condensation or by absorption in suitable media, with specific operation and purification of the maleic anhydride. The following examples will serve to provide a fuller understanding of the invention, but it is to be understood that these samples are given for illustrative purposes only and will not be interpreted as limiting the invention in any way. In the examples the terms "conversion", "selectivity" and "yield" are defined as follows: ##EQU1##